Procedures for preparing photographic images in silver by diffusion transfer principles are well known to the art. Thus, a silver diffusion transfer reversal process may provide a positive silver transfer image by development of the latent image provided by exposure of a photosensitive silver halide emulsion and, substantially contemporaneous with such development, a soluble silver complex is obtained, by reaction of a silver halide solvent with unexposed and undeveloped silver halide of the emulsion. The resultant soluble silver complex is, at least in part, transported in the direction of a suitable print-receiving element and the silver of the complex there precipitated to provide the requisite positive silver image formation.
The silver receptive stratum employed may be so constituted as to provide an unusually effective silver precipitating environment which causes the silver deposited therein, in comparison with negative silver developed in the silver halide emulsion, to possess an extraordinarily high covering power, that is, opacity per given mass of reduced silver; see Edwin H. Land, One Step Photography, Photographic Journal, section A, pp. 7-15, January 1950.
Specifically, to provide such environment, silver precipitation nuclei may be disposed within the silver receptive stratum in clusters possessing a diameter directly proportional to the mass of image silver to be deposited in situ by reduction. Such conformation can be employed to cause image silver to precipitate, in association with the silver precipitation nuclei clusters, with the required density and of a size directly related to the physical parameters of the clusters. The image silver thus precipitated in situ in galaxies of chosen physical parameters provides image conformation in which the elemental silver of the print-receiving element may possess a very high order of covering power, for example, five to fifteen or more times that of the negative elemental image silver in the silver halide element.
Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color, such as red, or green or blue, and by viewing the reversed or positive silver image formed by transfer to a transparent print-receiving element through the same or a similar screen which is suitably registered with the reversed positive image carried by the print-receiving layer.
As examples of suitable film structures for employment in additive color photography, mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,427; 3,615,428; 3,615,429; 3,615,426; and 3,894,871.
U.S. Pat. No. 3,536,488 is directed to photosensitive silver halide crystals or grains dispersed in an environment containing silver precipitating nuclei or agents which in the presence of a solvent developer composition cause exposed grains to be reduced to opaque structures smaller in presented area than the area of the same grains developed in an identical developer composition absent such precipitating nuclei. Silver image masses derived from exposed silver halide grains developed in accordance with U.S. Pat. No. 3,536,488, accordingly, possess low optical covering power as compared with the covering power provided by identical grains developed in the same solvent developer absent the presence of the precipitating environment. Specifically, the patent provides for the production of a direct positive silver image in which the mass distribution of silver is substantially uniform macroscopically and nonuniform microscopically, and in which the transmissiveness of silver image mass is a function of the quantity of actinic radiation which exposed the photosensitive silver halide. The exposed silver halide grains are reduced, in situ, as compact masses possessing low covering power simultaneously with reduction, in situ, of unexposed silver halide grains as a colloidal dispersion possessing high covering power. The direct positive silver image thus produced in situ possesses extraordinary high sharpness when compared with transfer processes in which unexposed silver halide grains are dissolved and transferred to the ultimate image-carrying site. U.S. Pat. No. 3,536,488 is incorporated by reference herein in its entirety.
As set forth in U.S. Pat. No. 2,698,236 the array of silver atoms precipitated in the image-receiving element is influenced by the size of the silver-precipitating nuclei. If the silver precipitating nuclei are relatively small, the silver deposited thereon will be of a corresponding size and, therefore, generally red in color which is undesirable. Clusters or galaxies of silver precipitating nuclei possessing a diameter directly proportional to the mass of individual silver particles to be precipitated therein are disposed in the image-receiving layer to cause silver to precipitate in association with silver precipitating nuclei clusters with a required density and of a size directly related to the physical parameters of the clusters, thus providing the desired black silver image.
It has also been found that silver precipitating nuclei disposed in a relatively thick layer, i.e., 1 micron or greater, also results in a redder image. While theories can be advanced for this phenomenon, the precise reason is not known.
Noble metal silver precipitating nuclei are known to the art. Copending application Ser. No. 69,282 filed Aug. 24, 1979 (common assignee) discloses a method of forming such nuclei by the reduction of a noble metal salt or complex. U.S. Pat. No. 3,647,440 also discloses noble metal silver precipitating nuclei obtained by reducing a metal salt in the presence of a protective colloid with a reducing agent having a standard potential more negative than -0.30.
Copending application Ser. No. 897,944, filed Apr. 4, 1978, (common assignee), now U.S. Pat. No. 4,209,330, is directed to a method of forming clusters of noble metal silver precipitating nuclei. Noble metal nuclei formed by the reduction of noble metal salts or complexes in colloidal suspension are formed into clusters by causing an instability in the colloid, e.g., by adjusting the pH of the colloid solution to at least about 2.6. If desired, the clusters may be employed at this point either by suitable separation techniques or by the addition of a bulking polymer to the colloid to impart stability to the clusters. If a separation technique is employed, the clusters may be allowed to floc, whereupon separation of the floc may be carried out by filtering and the like, and then redispersing the flocced particles in a suitable polymer binder with, preferably, sonification, and coating the thus-formed polymer/cluster mixture which may be coated on a support to provide an image-receiving element.
Such a method presents scale-up problems, and, in addition, the nuclei so produced must be used relatively soon after preparation to insure sensitometric uniformity in a product employing the nuclei.
A novel method for producing noble metal silver precipitating nuclei has now been found, which nuclei are particularly suited for use in the same layer as the photosensitive silver halide grains.